Apparatus for separation and purification of saturated hydrocarbon and method for separation and purification

ABSTRACT

An extractive distillation tower  4  supplied with a feedstock containing butadiene and a solvent and for distilling the feedstock to separate and purify the butadiene. The tower  4  includes sensors  32, 34  for detecting concentrations of specific impurities other than butadiene; a sensor for detecting a concentration of the butadiene; a sensor  30  for detecting a differential pressure between the top and bottom of the tower  4 ; a valve  48  for controlling the flow rate of part of fluid taken out from a bottom of the tower  4  and returned to the tower  4 ; a valve  23  for controlling a ratio of a solvent fed to the tower  4 ; a reflux ratio valve  28  for controlling the flow rate of part of remaining component of the feedstock taken out from a top of the tower  4  and refluxed; a heater  36  for controlling a bottom temperature of the tower  4 ; and a predictive control  60  for calculating forecasted values of the concentrations of the specific impurity and the butadiene after a time based on these sensors and controlling the system based on the forecasted values.

TECHNICAL FIELD

The present invention relates to an apparatus and method for separationand purification of unsaturated hydrocarbons, more particularly relatesto an apparatus and method for separation and purification ofunsaturated hydrocarbons which enable a target unsaturated hydrocarbonto be taken out stably at a predetermined concentration.

BACKGROUND ART

1,3-butadiene, isoprene, and other conjugated dienes are generallyseparated and purified as unsaturated hydrocarbons by extractivedistillation using an extraction solvent from a C₄ fraction or C₅fraction obtained by cracking naphtha and separating the ethylene,propylene, and other C₂ and C₃ hydrocarbons (Japanese Examined PatentPublication (Kokoku) No. 45-17405, Japanese Examined Patent Publication(Kokoku) No. 45-17411, Japanese Examined Patent Publication (Kokoku) No.47-41323, Japanese Unexamined Patent Publication (Kokai) No. 56-83421,etc.)

Normally, this extractive distillation is performed using an apparatuscomprised of an extractive distillation tower and stripping tower.Conjugated dienes, which dissolve relatively easily in the solventsincluded in the C₄ fraction or C₅ fraction, are taken out as mixtureswith the solvents from the bottom of the extractive distillation towerand sent to the stripping tower, where the conjugated dienes andsolvents are separated. The solvents are then returned to the extractivedistillation tower.

In the conventional separation and purification apparatus and method forconjugated dienes, the general practice has been to control the ratio ofthe extraction solvent fed to the extractive distillation tower, controlthe flow rate of part of the residual components of the feedstock takenout from the top of the extractive distillation tower (residuum offeedstock after conjugated dienes have been extracted) and reflux it tothe extractive distillation tower, control the bottom temperature of theextractive distillation tower, etc. to separate and purify a stablequality of conjugated dienes.

With such a conventional separation and purification apparatus andmethod of conjugated dienes, however, when the composition of thefeedstock fed to the extractive distillation tower varied, theconcentration of the target conjugated dienes taken out from the towervaried. Consequently, it was difficult to take out a stable quality ofconjugated dienes.

Note that to take out an extract of a high concentration and constantconcentration of conjugated dienes from the extractive distillationtower, it is preferable to return the extract taken out from the bottomof the extractive distillation tower to the extractive distillationtower and control the return ratio. If the return ratio to theextractive distillation tower, however, is not allowed to fluctuate inaccordance with the ratio of the extraction solvent, the bottomtemperature, the bottom pressure, the ratio of the feedstock fed, theconcentration of the conjugated dienes in the feedstock, etc., it is notpossible to maintain a constant concentration of the target butadiene,isoprene, or other conjugated dienes and concentration of other specificimpurities in the extractive distillation tower. Further, it is close toimpossible for an operator to manually handle this control procedure.Therefore, at the present time, priority is given to ease of operationeven if allowing the concentration of the conjugated dienes taken outfrom the extractive distillation tower to fluctuate somewhat. The returnratio is not controlled, but the flow rate to the next process iscontrolled and the surplus is returned. Therefore, there was a largefluctuation in the conjugated dienes taken out from the extractivedistillation tower. In particular, if there is a large fluctuation inconcentration of the conjugated dienes taken out in the first extractivedistillation tower used for the separation and purification apparatus ofthe conjugated dienes, increasing the purity of the conjugated dienes inthe subsequent processes becomes difficult and stably obtaining highpurity conjugated dienes becomes difficult.

DISCLOSURE OF THE INVENTION

An object of the present invention is to provide an apparatus and methodfor separation and purification of unsaturated hydrocarbons which enablea target conjugated diene or other unsaturated hydrocarbons to be stablytaken out at a predetermined concentration regardless of variations inthe components of the feedstock or fluctuations in the ratio of thefeedstock fed.

To achieve this object, the apparatus for separation and purificationapparatus of an unsaturated hydrocarbon of the present inventioncomprises an extractive distillation tower fed with feedstock containingunsaturated hydrocarbons and a solvent and distilling the feedstock toseparate and purify a target unsaturated hydrocarbon; an impurityconcentration detecting means for detecting a concentration of aspecific impurity other than the target unsaturated hydrocarbon at theextractive distillation tower or another tower connected to theextractive distillation tower (the extractive distillation tower, etc.);a target material concentration detecting means for detecting aconcentration of the target unsaturated hydrocarbon at the extractivedistillation tower or another tower connected to the extractivedistillation tower; a return ratio control means for controlling a flowrate of part of a fluid including the target unsaturated hydrocarbontaken out from a bottom of the extractive distillation tower andreturned to the extractive distillation tower; a solvent ratio controlmeans for controlling a ratio of the solvent fed to the extractivedistillation tower; a reflux ratio control means for controlling a flowrate of part of a residual component of the feedstock taken out from atop of the extractive distillation tower (meaning the residuum of thefeedstock after the unsaturated hydrocarbons has been extracted, butalso including some unsaturated hydrocarbons, same below) and refluxedto the extractive distillation tower; a bottom temperature control meansfor controlling a bottom temperature of the extractive distillationtower; and a predictive control means for calculating forecasted valuesof the concentration of the specific impurity and the concentration ofthe target unsaturated hydrocarbon after a predetermined time based onvalues detected by the impurity concentration detecting means and targetmaterial concentration detecting means and controlling the return ratiocontrol means, solvent ratio control means, reflux ratio control means,and bottom temperature control means based on the forecasted values.

Further, the method for separation and purification of an unsaturatedhydrocarbon according to the present invention comprises the steps offeeding a feedstock containing a target unsaturated hydrocarbon and asolvent to an extractive distillation tower; detecting a concentrationof a specific impurity other than the target unsaturated hydrocarbon atthe extractive distillation tower or another tower connected to theextractive distillation tower; detecting a concentration of the targetunsaturated hydrocarbon at the extractive distillation tower or anothertower connected to the extractive distillation tower; controlling areturn flow rate of part of a fluid containing the target unsaturatedhydrocarbon taken out from a bottom of the extractive distillation towerand returned to the extractive distillation tower; controlling a ratioof the solvent fed to the extractive distillation tower; controlling areflux flow rate of part of a residual component of the feedstock takenout from a top of the extractive distillation tower and refluxed to theextractive distillation tower; controlling a bottom temperature of theextractive distillation tower; and calculating forecasted values of theconcentration of the specific impurity and the concentration of thetarget unsaturated hydrocarbon after a predetermined time based onvalues detected by the impurity concentration detecting step and targetmaterial concentration detecting step and controlling the return flowrate, the ratio of the solvent, the reflux flow rate, and the bottomtemperature based on the forecasted values.

The solvent used in the present invention may be dimethylformamide,diethylformamide, dimethylacetomide, and other N-alkyl substituted lowerfatty acid amides, furfural, N-methylpyrrolidone, formylmorpholine,β-methoxypropionitrile, and other solvents used for extractivedistillation of diolefins from hydrocarbon fractions for example. Thesesolvents may be used alone or may be used in mixtures of two or moretypes. Further, to adjust the boiling point, suitable amounts of water,methanol, etc. may be mixed. Further, it is also possible to jointly usepolymerization inhibitors to inhibit polymerization of the diolefins andacetylenes, antioxidants, defoaming agents, etc.

The extraction medium (solvent) is preferably fed to the extractivedistillation tower from an extractive distillation medium feed meansprovided at a position higher than the position feeding the petroleumfraction containing the unsaturated hydrocarbons in the extractivedistillation tower (petroleum fraction feed means).

Further, the polymerization inhibitor may be continuously fed from aposition higher than the feed position of the extraction medium. As theposition higher than the extraction medium feed position, for example,mention may be made of the side of the extractive distillation towerhigher than the extraction medium feed position or the inlet or outletof the condenser of the top of the extractive distillation tower. Amongthese, provision at the inlet of the top condenser is preferable in thatit enables the production of polymers inside the condenser to besuppressed and enables the production of polymers even in processesafter the separator to be suppressed. The polymerization inhibitor ispreferably one which stops or suppresses polymerization by a chaintransfer reaction, in particular, a lower alkylhydroxylamine.

The feedstock (petroleum fraction) used in the present inventioncontains unsaturated hydrocarbons. The petroleum fraction normally isobtained by cracking naphtha. As the petroleum fraction, there are forexample a C₂ fraction containing mainly C₂ hydrocarbons, a C₃ fractioncontaining mainly C₃ hydrocarbons, a C₄ fraction containing mainly C₄hydrocarbons, and a C₅ fraction containing mainly C₅ hydrocarbons. Amongthese, a fraction increased in the concentration of the unsaturatedhydrocarbons due to the extractive distillation etc. is preferred.Further, a fraction containing a large amount of conjugated dienes asunsaturated hydrocarbons is preferred. In particular, a C₄ fractioncontaining a large amount of butadiene or a C₅ fraction containing alarge amount of isoprene is preferred.

The apparatus and method of the present invention are effective whenapplied to the case of trying to obtain a concentration of anunsaturated hydrocarbon in the petroleum fraction of normally at least90 percent, preferably at least 95 percent (specifically, extracting anddistilling the petroleum fraction to increase the concentration of theunsaturated hydrocarbon).

According to the apparatus and method of the present invention, it ispossible to stably take out a target unsaturated hydrocarbon at apredetermined concentration regardless of variations in the feedstockcomponents.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be described below with reference toembodiments shown in the drawings.

FIG. 1 is a schematic view of the overall configuration of a separationand purification apparatus for conjugated dienes;

FIG. 2 is a schematic view of a method of control of a first extractivedistillation tower shown in FIG. 1;

FIG. 3 is a flow chart of the method of control of a predictive controlmeans shown in FIG. 2;

FIG. 4 is a graph of the relationship of the measurement data andcontrol parameters; and

FIG. 5 is a schematic view of the overall configuration of a separationand purification apparatus for butenes.

BEST MODE FOR CARRYING OUT THE INVENTION First Embodiment

In the present embodiment, the explanation will be given of the processof separation and purification of conjugated dienes from a C₄ fractionor C₅ fraction containing conjugated dienes as unsaturated hydrocarbons.

As shown in FIG. 1, the C₄ fraction or C₅ fraction is first vaporized atan evaporation tower 2 and fed to a first extractive distillation tower4. Further, the solvent is fed to a stage higher than the C₄ fraction orC₅ fraction feed position of the first extractive distillation tower 4.The solvent containing the conjugated dienes is fed from the bottom ofthe first extractive distillation tower 4 to a position several stagesdown from the top of a stripping tower 8. In the tower, the conjugateddienes and solvent are separated. The bottom temperature of the tower isnormally controlled to become the boiling point of the solvent at atower pressure of 0.5 to 5 atm. The conjugated dienes are taken out fromthe top of the stripping tower 8. Part is sent to a second extractivedistillation tower 12 where it is purified, while the remainder isreturned to the first extractive distillation tower 4. Solvent ofnormally 100 to 200° C. is taken out from the bottom of the strippingtower 8.

In the present embodiment, by detecting the change in the concentrationof the impurity near the bottom of the first extractive distillationtower and the change of the concentration of the conjugated dienes inthe gas discharged from the top of the first extractive distillationtower and controlling the ratio of the solvent fed to the firstextractive distillation tower 4, controlling the return ratio from thestripping tower 8 to the first extractive distillation tower 4,controlling the reflux ratio at the top of the first extractivedistillation tower 4, and controlling the bottom temperature of thefirst extractive distillation tower 4 in accordance with these changes,it is possible to extract a constant concentration of conjugated dienes.

Below, a detailed explanation will be given of the process of separationand purification of butadiene from a C₄ fraction as an example.

As shown in FIG. 1, a feedstock (C₄ component in naphtha) C4F containingbutadiene is fed to the evaporation tower 2 where the feedstock C4F isvaporized. In the evaporation tower 2, the C4F is vaporized by holdingthe tower temperature at preferably 20 to 80° C., more preferably 40 to80° C., and holding the tower pressure at an absolute pressure ofpreferably 2 to 8 atm, more preferably 4 to 6 atm.

The feedstock C4F vaporized at the evaporation tower 2 is next fed tothe first extractive distillation tower 4. The first extractivedistillation tower 4 is fed with a solvent together with the vaporizedfeedstock C4F. The ratio of the solvent fed to the first extractivedistillation tower is controlled as explained later, but in general thesolvent is fed to 100 to 1000 parts by weight, more preferably 200 to800 parts by weight, with respect to 100 parts by weight of thefeedstock C4F. The temperature of the solvent is preferably low sincethe solubility is high, but preferably is 10 to 100° C., more preferably20 to 60° C. since it affects the internal temperature of the firstextractive distillation tower 4 or the change of the reflux ratio.

The solvent is not particularly limited so long as it enablesdissolution and extraction of butadiene as one example of conjugateddienes, but specifically acetone, methylethylketone, dioxane, isoprenecyclic sulfone, acetonitrile, alcohol, glycol, N-methyloldamine,N-ethylsuccinic acid imide, N-methylpyrrolidone, N-methyl-2-pyrrolidone,hydroxylethylpyrrolidone, N-methyl-5-methylpyrrolidone, furfural,2-heptenone, dimethylformamide, dimethylacetamide, N,N-dimethylacetoneacetic acid amide, morpholine, N-formylmorpholine,N-methylmorpholin-3-one, sulforane, methylcarbitol, tetrahydrofuran,aniline, N-methyloxazolidone, N-methylimidazole,N,N′-dimethylimidazolin-2-one, 1-oxo-1-methylphosphorin,methylcyanoacetate, ethylacetoacetate, ethylacetate, malonic aciddimethylester, propylene carbonate, methylcarbitol, triethyl phosphate,diethylene glycol monomethyl ether, dimethyl sulfoxide, γ-butyrolactone,etc. may be mentioned. In the present embodiment, as the extractionmedium, among these, amide compounds, in particular dimethylformamide,are preferable.

The extraction medium is fed to the first extractive distillation tower4 from an extraction medium feed stage provided at a position higherthan the stage feeding the petroleum fraction containing the conjugateddienes (feedstock BBF) in the first extractive distillation tower 4(petroleum fraction feed stage).

At the top of the first extractive distillation tower 4 shown in FIGS. 1and 2, the gas having a volatility (solubility) of at least butadiene isseparated, the raffinate C4R of the feedstock C4F from which thebutadiene component has been separated is taken out, and a highconcentration butadiene extract is taken out from the bottom of thetower by controlling the bottom pressure of the first extractivedistillation tower 4 to an absolute pressure of preferably 1 to 10 atm,more preferably 5 to 7 atm, and the bottom temperature to preferably 100to 160° C., more preferably 110 to 130° C.

The amount of the C₄ fraction dissolved in the solvent taken out fromthe bottom of the first extractive distillation tower 4 is determined bythe solvent ratio, temperature, and pressure at the bottom of the tower.Therefore, to take out a constant concentration butadiene extract fromthe bottom of the first extractive distillation tower 4, it is necessaryto control the solvent ratio at the bottom of the first extractivedistillation tower 4, the reflux ratio of the top, the bottomtemperature, etc. Further, to increase the concentration of thebutadiene extract solution taken out from the bottom of the firstextractive distillation tower 4, as mentioned later, it is necessary toreturn the extract taken out from the bottom of the first extractivedistillation tower 4 or, in accordance with need, part from which thesolvent has been removed through the stripping tower 8, to the firstextractive distillation tower 4. In the present embodiment, as explainedlater, the return ratio of the extract taken out from the bottom of thefirst extractive distillation tower 4 to the first extractivedistillation tower 4 is also controlled.

The C4 raffinate taken out from the top of the first extractivedistillation tower 4 is sent to a not shown residual component tank.Part of the residual gas BBR is condensed at a not shown condenser andrefluxed by returning it to the top of the first extractive distillationtower 4. The reflux ratio of the residual gas BBR is also controlled asexplained later.

At the bottom of the first extractive distillation tower 4 shown inFIGS. 1 and 2, an extract containing a high concentration of the targetbutadiene is taken out and sent to the stripping tower 8. In thestripping tower 8, the bottom pressure is held at an absolute pressureof 1 to 3 atm and the bottom temperature is held at 150 to 200° C. Thesolvent is separated from the extract and discharged from the bottom ofthe tower. At the top of the stripping tower 8, a stripped gascontaining a large amount of butadiene from which the solvent has beenseparated is produced. When condensing part of the stripped gas in thecondenser, the condensed part is refluxed by returning it to the top ofthe stripping tower 8. Part of the uncondensed part is returned througha compressor 10 to the first extractive distillation tower 4, while theremainder is sent to a second extractive distillation tower 12. Whencondensing all of the stripped gas at the condenser, part of thecondensed liquid is refluxed by returning it to the top of the strippingtower 8, part of the remainder is returned through the compressor 10 tothe first extractive distillation tower 4, and the rest is sent to thesecond extractive distillation tower 12. In this way, what is returnedto the first extractive distillation tower 4 is vapor in case ofhandling with C4 fraction, or liquid in case of C5 fraction. In bothcases, however, the return ratio is controlled as explained later.

In the second extractive distillation tower 12, impurities having avolatility (solubility) of not more than butadiene are separated fromthe bottom of the tower. At the top of the tower, gas containing a highconcentration of butadiene is taken out by holding the bottom pressureat an absolute pressure of 3 to 6 atm and holding the bottom temperatureat 100 to 150° C. An extract containing a large amount of impuritiesseparated from the bottom of the second extractive distillation tower 12is led to the first stripping tower 13. At the first stripping tower 13,the bottom pressure is held at an absolute pressure of 1 to 3 atm andthe bottom temperature is held at 120 to 180° C. The butadiene isseparated from the extract and the stripped gas containing the butadieneis returned to the inlet of the condenser of the stripping tower 8. Theliquid at the bottom of the second extractive distillation tower 12 issent to the second stripping tower 14. At the second stripping tower 14,the bottom pressure is held at an absolute pressure of 1 to 3 atm andthe bottom temperature is held at 150 to 200° C. The solvent isseparated from the extract, exhausted from the bottom of the tower, andreused. The stripped gas is exhausted from the top of the tower.

The distillation gas containing a large amount of butadiene taken outfrom the top of the second extractive distillation tower 12 issuccessively sent to a topping tower 16 and a tailing tower 18. At thetopping tower 16, the methylacetylene impurity having a lower boilingpoint than butadiene is removed by making the bottom pressure 3 to 7 atmand making the bottom temperature 30 to 60° C. Further, at the tailingtower 18, the impurities having a higher boiling point than butadiene,for example, cis-2-butene, 1,2-butadiene, and ethylacetylene, areremoved by making the bottom pressure 3 to 7 atm and the bottomtemperature 40 to 70° C. In the present embodiment, the concentration ofthe butadiene in the finally obtained extract becomes at least 99percent.

Next, an explanation will be given of the control apparatus and controlmethod of the first extractive distillation tower 4 according to thepresent embodiment referring mainly to FIGS. 2 to 4.

As shown in FIG. 4, a feedstock feed line 20 to which a feedstock(petroleum fraction) C4F containing butadiene is fed is connected to anintermediate stage of the first extractive distillation tower 4. Thefeedstock feed line 20 has a feedstock flowmeter 21 attached to it tomeasure the flow rate of the feedstock fed to the first extractivedistillation tower 4. The feedstock flow rate data measured by thefeedstock flowmeter 21 is input to a predictive control means 60. Thepredictive control means 60 is comprised of a specific electricalcircuit having a memory circuit, a general use PC, general use computer,large computer, etc. and stores a program for the later mentionedcontrol. Note that instead of a program for control discussed later, itis also possible to provide a logic circuit operating in that way.

In the first extractive distillation tower 4, a solvent feed line 22 isconnected to the top plate side of the feedstock feed line 20 and feedsthe solvent for extraction of the butadiene to the inside of the firstextractive distillation tower 4. The solvent feed line 22 has a solventflow rate control valve 23 attached to it. This controls the flow rateto the first extractive distillation tower 4 based on the output signalfrom the predictive control means 60. The method of control will beexplained below.

A residual gas exhaust line 24 is connected to the top of the firstextractive distillation tower 4 and exhausts part of the residual gasremaining after extraction of the butadiene from the feedstock in thefirst extractive distillation tower 4 (however, containing somebutadiene) to a not shown residual component tank. A reflux line 26 isalso connected to the exhaust line 24 and refluxes part of the residualgas taken out at the exhaust line 24 to the top of the first extractivedistillation tower 4.

The exhaust line 24 near the top of the first extractive distillationtower 4 has attached to it a target material concentration sensor 25 fordetecting the concentration of butadiene at the top of the tower (targetmaterial concentration detecting means) and a top pressure sensor 27 formeasuring the pressure inside the top of the tower. As the targetmaterial concentration sensor 25, for example, a gas chromatograph maybe used. As the pressure sensor 27, a general use pressure sensor may beused. The output signals of these sensors 25 and 27 are input to thepredictive control means 60.

A control valve 28 serving as the reflux ratio control means is attachedto the reflux line 28. The opening degree of the control valve 28 iscontrolled by the output signal from the predictive control means 60 soas to control the reflux flow rate.

In the example shown in FIG. 2, a differential pressure sensor 30 isattached to the first extractive distillation tower 4 as a differentialpressure detecting means for detecting the pressure difference betweenthe inside of the top of the tower and the inside of the bottom of thetower. In this example, the top-bottom differential pressure datadetected by the differential pressure sensor 30 is input to thepredictive control means 60. Predictive control by the change of thepressure difference contributes only a little to the stability of thecomposition of the extract, but contributes a lot to the safety, sopredictive control is preferable.

First and second impurity concentration sensors (impurity concentrationdetecting means) 32 and 34 for measuring the concentration of thetrans-2-butene and cis-2-butene and other impurities present at theseventh stage are attached to the seventh stage from the bottom of thefirst extractive distillation tower 4. The first impurity concentrationsensor 32 detects the concentration of the cis-2-butene, while thesecond impurity concentration sensor 34 measures the concentration ofthe trans-2-butene. These concentration sensors 32 and 34 are notparticularly limited so long as they can detect the concentrations, butfor example are comprised of gas chromatographs. The data on theconcentrations detected by these concentration sensors 32 and 34 areinput to the predictive control means 60.

Note that the positions of the first sensor and second sensor are notlimited to the seventh stage from the bottom of the first extractivedistillation tower 4. For example, they may be at the 7th stage from thebottom, on the stripped gas line 44 from the stripping tower 8, or atthe condensate line 51 as well. While the concentration data will notmatch at these locations, there is a strong correlation in the amountsof change of the concentrations. If continuously measuring theconcentrations at any of these locations, it is possible to accuratelyjudge the concentrations at the other locations. Further, since thepredictive control means uses the concentration data converted to dataon the change of concentration, if the change in concentration at theselocations can be accurately measured, accurate predictive control ispossible.

A bottom heater 36 is provided as the bottom temperature control meansat the bottom of the first extractive distillation tower 4. The bottomtemperature is controlled based on the output signal corresponding tothe data on the change of concentration from the predictive controlmeans 60. The bottom temperature, as explained above, is generally heldat 100 to 160° C., but in the present embodiment the bottom temperatureis controlled based on the output signal from the predictive controlmeans 60 in that temperature range. The heat source for the bottomheater 36 is not limited to steam. Hot water, a heat medium, etc. mayalso be mentioned. Of course, the bottom heater 36 can be controlled intemperature.

An extract containing a high concentration of butadiene present at thebottom of the tower 4 (containing solvent) is sent to the strippingtower 8 through line 38. At the stripping tower 8, as explained above,the solvent is separated from the extract and exhausted from the bottom.At the top of the stripping tower 8, stripped gas containing a largeamount of butadiene from which the solvent has been separated isproduced. This gas is sent from the top of the tower through a line 44to the second extractive distillation tower 12 by a compressor 10. Areturn line 46 is connected to the line 44. The return line 46 isconnected to a stage near the bottom of the first extractivedistillation tower 4. The stripped gas containing a large amount ofbutadiene carried through the line 44 or its condensate is returned tothe inside of the first extractive distillation tower

The return line 46 is fitted with a return flowmeter 50 for detectingthe flow rate in the line and a return flow rate control valve 48 forcontrolling the flow rate of the fluid flowing through the line. Thecontrol valve 48 is controlled in accordance with an output signal fromthe predictive control means 60 and controls the flow rate of the fluidreturned inside the first extractive distillation tower 4 through thereturn line 46. The control valve 48 and the flowmeter 50 correspond tothe return ratio control means. When returning part of the stripped gasto the first extractive distillation tower 4 as a gas, the remainder ofthe stripped gas is sent to the second extractive distillation tower 12while controlling the value of the pressure sensor 52 by the controlvalve 56. When returning the condensate of the stripped gas to the firstextractive distillation tower 4, the remainder of the condensate is sentto the second extractive distillation tower 12 while controlling theliquid level of a condensate drum by the control valve 56.

In the present invention, the predictive control means is notparticularly limited, but as an example an explanation will be given ofa method of control using the predictive control means 60 shown in FIG.2 based on FIG. 3 and FIG. 4.

When the control starts at step S1 shown in FIG. 3, at step S2, thepredictive control means 60 shown in FIG. 2 reads the data CVi (i=1 to4). CV1 is the data of the concentration of the cis-2-butene detected bythe impurity concentration sensor 32 at the seventh stage of theextractive distillation tower 2 shown in FIG. 2. CV2 is the data of theconcentration of the trans-2-butene detected by the impurityconcentration sensor 34 at the seventh stage of the extractivedistillation tower 2 shown in FIG. 2. CV3 is the data of theconcentration of the butadiene detected by the target materialconcentration sensor 25 attached to the top of the extractivedistillation tower 2 shown in FIG. 2. CV4 is the top-bottom differentialpressure data detected by the differential pressure sensor 30 of theextractive distillation tower 2 shown in FIG. 2. While not dataessential for control as mentioned above, if this is used for control,the safety becomes higher, so use is preferred.

Next, at step S3 shown in FIG. 3, the data CV1 to CV4 after t secondsare forecast from a control model stored in the predictive control means60 shown in FIG. 2 based on the data CV1 to CV4 and those values madeFCV1 to FCV4. The control model stored in the predictive control means60 is a control model determined based on the following formula. A modelof the relation between the measurement data CV1 to CV4 in an actualfirst extractive distillation tower 4 and the control parameters (returnratio MV1, solvent ratio MV2, reflux ratio MV3, and bottom temperatureMV4) is produced by this formula.${G_{ij}(S)} = {\frac{a_{ij}}{1 + {b_{ij} \cdot S}}\quad ^{{- c_{ij}} \cdot s}}$

where, i=1 to 4 and

j=1 to 4

In the above formula, G_(ij) shows the transfer function between CVi(i=1 to 4) and MVj (j=1 to 4), S is the parameter of a Laplacetransform, and a_(ij), b_(ij), and c_(ij) are values inherent to theprocesses corresponding to combinations of CVi (i=1 to 4) and MVj (j=1to 4) and are found from the results of a step test. Note that a steptest changes the MVj (j=1 to 4) in steps of any amount to find theresponse data of the CVi (i=1 to 4).

Using the control model established by this formula, at step S3 shown inFIG. 3, the data CV1 to CV4 after a predetermined time (t seconds) isforecast and the values used as FCV1 to FCV4. Note that “after tseconds” is not particularly limited, but is for example after 600 to3600 seconds.

Next, at step S4 shown in FIG. 3, the difference Ai (i=1 to 4) betweenthe target value PCVi preset for every data CV1 to CV4 and theforecasted value FCVi is calculated. Next, at step S5, it is confirmedif the difference Ai is in a predetermined range from -αi (minusallowable value) to +αi (plus allowable value). If the difference Ai isin the predetermined range, it means that the forecasted value FCVi ofthe CVi after t seconds is in the allowable range. Note that theallowable value αi is determined for each CVi. While not particularlylimited, it is about 1 to 10 percent of the target value PCVi.

If all of the differences Ai are allowable values at step S5, theforecasted values FCVi of the CVi after t seconds are in the allowablerange, so the control parameters MV1 to MV4 are maintained in theircurrent states and the steps after step S2 are repeated. If even one ofthe differences Ai is outside of the allowable range at step S5, itmeans that the corresponding forecasted value FCVi is outside of theallowable range, so the routine proceeds to step S6, where the currentsettings of the control parameters MVi are changed so as to change theforecasted value FCVi deviating from the allowable range in a directionentering the allowable range. For example, when desiring to control aforecasted value FCVi deviating from the allowable range in a directionlowering the value, the current settings of the control parameters MViare changed in the directions of the arrows shown in FIG. 4. In FIG. 4,the upward facing arrows mean raising the current settings of thecontrol parameters MVi.

For example, when the forecasted concentration value FCV1 of the seventhstage cis-2-butene corresponding to the data CVI detected by theconcentration sensor 32 shown in FIG. 2 rises out of the predeterminedrange, the forecasted concentration value FCV1 of the seventh platecis-2-butene is lowered by changing the current settings of the controlparameters MVi as follows: That is, the predictive control means 60shown in FIG. 2 is used to operate the control valve 48 to increase thereturn ratio MV1 to the first extractive distillation tower 2 throughthe return line 46. Further, the predictive control means 60 shown inFIG. 2 is used to control the control valve 23 to reduce the ratio ofthe solvent MV2 fed to the first extractive distillation tower 4 throughthe solvent feed line 22. Further, the predictive control means 60 shownin FIG. 2 is used to control the control valve 28 of the reflux line 26to reduce the reflux ratio MV3. Further, the predictive control means 60shown in FIG. 2 is used to control the heater 36 to increase the bottomtemperature MV4.

Similarly, when the forecasted value of the concentration FCV2 of theseventh-stage trans-2-butene corresponding to the data CV2 detected bythe concentration sensor 34 shown in FIG. 2 rises outside of thepredetermined range, the FCV2 is lowered by changing the currentsettings of the control parameters MVi in accordance with the directionsof the arrows shown in FIG. 4. Similarly, when the forecasted value ofthe concentration FCV3 of the butadiene at the top of the towercorresponding to the data CV3 detected by the concentration sensor 25shown in FIG. 2 rises outside of the predetermined range, the FCV3 islowered by changing the current settings of the control parameters MViin accordance with the directions of the arrows shown in FIG. 4. Whenthe forecasted top-bottom differential pressure value FCV4 correspondingto the data CV4 detected by the differential pressure sensor 30 shown inFIG. 2 rises outside of the predetermined range, it is preferable interms of safety to lower this FCV4 by changing the current settings ofthe control parameters MVi in accordance with the directions of thearrows shown in FIG. 4. Note that when the forecasted values FCV1 toFCV4 corresponding to the data CV1 to CV4 drop outside of thepredetermined ranges, the control parameters MVi are controlled by thepredictive control means 60 in directions opposite to the arrows shownin FIG. 4.

By using the method of control of the first extractive distillationtower 4 according to the present embodiment, it is possible to reducethe variation in the concentration CV1 of the seventh stage cis-2-buteneto about 0.63 percent (min. 8.50-max. 9.13%). Note that in conventionalcontrol, the variation in the concentration of the seventh stagecis-2-butene was 1.29 percent (min. 7.79-max. 9.08%). Further, accordingto the method of the present embodiment, it is possible to reduce thevariation in the concentration CV2 of the seventh stage trans-2-buteneto about 0.32 percent (min. 1.35-max. 1.67%). Note that in conventionalcontrol, the variation in the concentration CV2 of the seventh stagetrans-2-butene was 0.54 percent (min. 1.28-max. 1.82%). Further,according to the method of the present embodiment, the variation in theconcentration CV3 of the butadiene at the top of the tower can bereduced to about 0.21 percent (min. 0.19-max. 0.40%). Note that inconventional control, the variation in the concentration CV3 of thebutadiene at the top of the tower was 0.29 percent (min. 0.13% -max.0.42%).

According to the apparatus and method of the present embodiment, bysuppressing the variations of the concentrations CV1 and CV2 of thecis-2-butene and trans-2-butene as impurities near the bottom of thefirst extractive distillation tower 4 and suppressing the variations ofthe concentration of the butadiene at the top of the tower, it ispossible to stabilize the concentration of butadiene included in theextract taken out from the bottom. As a result, increasing the purity ofthe conjugated butadienes in the subsequent process becomes easy, andhigh purity conjugated butadienes can be stably obtained.

Second Embodiment

In the present embodiment, the process of separation and purification ofunsaturated hydrocarbons other than conjugated dienes will be explained.In the present embodiment, the raffinate C4R produced as a byproduct inthe process of separation and purification of the conjugated dienes fromthe C₄ fraction or the C₅ fraction of the first embodiment shown in FIG.1 (residual gas of feedstock C4F from which butadiene has beenseparated) is used as the feedstock and butenes are separated andpurified.

The raffinate C4R contains 30 to 80 percent butenes. The rest consistsof butanes. As shown in FIG. 5, the residual gas BBR is fed to theevaporation tower 101 where it is vaporized and then is fed to theextractive distillation tower 102.

Further, a solvent is fed to a stage higher than the position of feedingthe BBR of the first extractive distillation tower 102. The solventcontaining the butenes is taken out from the bottom of the extractivedistillation tower 102. Almost all of the butanes contained in the BBRare exhausted from the top of the tower. The solvent containing thebutenes taken out from the bottom of the extractive distillation tower102 is fed to a position several stages lower than the top of astripping tower 103. In the stripping tower, the butenes and solvent areseparated.

The bottom temperature of the extractive distillation tower 102 iscontrolled to become the boiling point of the solvent at the towerpressure, that is, normally 0.5 to 5 atm. Butenes are taken out from thetop of the stripping tower 103. Part is sent to the solvent recoverytower 105, while the remainder is returned to the extractivedistillation tower 102. Normally solvent of 100 to 200° C. is taken outfrom the bottom of the stripping tower 103. At the solvent recoverytower 105, butenes are recovered from the top of the tower, while amixture of the solvent and butenes is returned from the bottom of thetower to the bottom of the extractive distillation tower 102. Thebutenes recovered from the tower 105 are generally a mixture ofn-butene, isobutene, 1-butene, and 2-butene. When separation andpurification of these are necessary, since almost all of the butaneshard to be separated and purified are removed, separation andpurification can be easily performed by a normal distillation operation.

In the present embodiment, by detecting the change in the concentrationof the impurities near the bottom of the extractive distillation tower102 and the change of the concentration of the butenes in the butane gasdischarged from the top of the extractive distillation tower 102 andcontrolling the ratio of the solvent fed to the extractive distillationtower 102, controlling the return ratio from the stripping tower 103through the solvent recovery tower 105 to the extractive distillationtower 102 controlling the reflux ratio at the top of the extractivedistillation tower 102, and controlling the bottom temperature of theextractive distillation tower 102 in accordance with these changes, itis possible to extract a constant concentration of butenes. In thepresent embodiment, the concentration of butenes in the finally obtainedextract is at least 99 percent.

The control apparatus and control method of the extractive distillationtower 102 according to the present embodiment is similar to the case ofthe first extractive distillation tower 4 shown in FIG. 2 except thatthe feedstock fed is changed from BBF to BBR, the gas exhausted from thetop of the tower is changed from BBR to a gas containing butenes, theimpurities detected as CV1 and CV2 are changed from cis-2-butene andtrans-2-butene to n-butane and other butanes, and the extract taken outfrom the bottom of the extractive distillation tower 102 is changed fromconjugated dienes to butenes. The common description will be omitted.

According to the apparatus and method of the present embodiment, bysuppressing the variations of the concentrations CV1 and CV2 of thebutane impurities near the bottom of the extractive distillation tower102 and suppressing the variations of the concentration of the butenesat the top of the tower, it is possible to stabilize the concentrationof butenes included in the extract taken out from the bottom. As aresult, increasing the purity of the butenes in the subsequent processbecomes easy, the loss of the butenes at the top of the tower can besuppressed, and high purity butenes can be stably obtained.

Other Embodiments

Note that the present invention is not limited to the above embodimentsand may be modified in various ways within the scope of the invention.

For example, in the first embodiment, two impurity concentration sensors32 and 34 were used as the impurity concentration detecting means in thefirst extractive distillation tower 4, but in the present invention itis also possible to use either of the impurity concentration sensors 32or 34 and omit the other one. This because there is a correlationbetween the concentration of the cis-2-butene and the concentration ofthe trans-2-butene detected by these concentration sensors 32 and 34 andby detecting and suppressing the concentration of either of these, it ispossible to also suppress fluctuations in concentration of the other. Ifomitting one, however, it is preferable to omit the impurityconcentration sensor 32 for detecting the concentration of thecis-2-butene. This is because the cis-2-butene can be removed in aprocess after the first extractive distillation tower 4. Note that thesame applies to the second embodiment.

Further, in the first embodiment, the concentration of the targetbutadiene was detected at the top of the extractive distillation tower4, but in the present invention the location is not limited to the topof the tower. Another location is also possible. However, detection atthe top of the tower is most preferable. The same applies to the secondembodiment.

What is claimed is:
 1. A separation and purification apparatus for anunsaturated hydrocarbon comprising; an extractive distillation towersupplied with feedstock containing unsaturated hydrocarbon and a solventfor distilling the feedstock to separate and purify a target unsaturatedhydrocarbon; an impurity concentration detecting means for detecting aconcentration of a specific impurity other than the target unsaturatedhydrocarbon at the extractive distillation tower or another towerconnected to the extractive distillation tower; a target unsaturatedhydrocarbon concentration detecting means for detecting a concentrationof the target unsaturated hydrocarbon at the extractive distillationtower or another tower connected to the extractive distillation tower; areturn ratio control means for controlling a flow rate of part of afluid including the target unsaturated hydrocarbon taken out from abottom of the extractive distillation tower and returned to theextractive distillation tower; a solvent flow control means forcontrolling a flow rate of the solvent fed to the extractivedistillation tower; a reflux ratio control means for controlling a flowrate or part of a residual component of the feed stock taken out from atop of the extractive distillation tower and refluxed to the extractivedistillation tower, a bottom temperature control means for controlling abottom temperature of the extractive distillation tower; and apredictive control means for calculating forecasted values of theconcentration of the specific impurity and the concentration of thetarget unsaturated hydrocarbon based on values detected by the impurityconcentration detecting means and target unsaturated hydrocarbonconcentration detecting means and controlling the return ratio controlmeans, solvent ratio control means, reflux ratio control means, andbottom temperature control means based on the forecasted values.
 2. Theseparation and purification apparatus for an unsaturated hydrocarbon asset forth in claim 1, further comprising a differential pressuredetecting means for detecting a differential pressure between a top andbottom of said extractive distillation tower, wherein the predictivecontrol means further calculates forecasted values of the concentrationof the specific impurity and the concentration of the target unsaturatedhydrocarbon based on values detected by the impurity concentrationdetecting means, target unsaturated hydrocarbon concentration detectingmeans, and differential pressure detecting means and controls the returnratio control means, solvent ratio control means, reflux ratio controlmeans, and bottom temperature control means based on the forecastedvalues.
 3. The separation and purification apparatus for an unsaturatedhydrocarbon as set forth in claim 1 or 2, wherein a solvent feedingmeans for feeding said solvent to said extractive distillation tower isprovided at a position higher than a petroleum fraction feed means forfeeding a petroleum fraction containing said unsaturated hydrocarbon. 4.The separation and purification apparatus for an unsaturated hydrocarbonas set forth in claims 1 or 2, further comprising an evaporation towerfor vaporizing a petroleum fraction containing said unsaturatedhydrocarbon before being fed to said extractive distillation tower. 5.The separation and purification apparatus for an unsaturated hydrocarbonas set forth in claims 1 or 2, further comprising a stripping tower fedwith a fluid containing the target unsaturated hydrocarbon taken outfrom the bottom of said extractive distillation tower and separating theunsaturated hydrocarbon and solvent.
 6. A method for separation andpurification of an unsaturated hydrocarbon comprising the steps of:supplying a feedstock containing a target unsaturated hydrocarbon and asolvent to an extractive distillation tower; detecting a concentrationof a specific impurity other than the target unsaturated hydrocarbon atthe extractive distillation tower or another tower connected to theextractive distillation tower; detecting a concentration of the targetunsaturated hydrocarbon at the extractive distillation tower or anothertower connected to the extractive distillation tower; controlling areturn flow rate of part of a fluid containing the target unsaturatedhydrocarbon taken out from a bottom of the extractive distillation towerand returned to the extractive distillation tower; controlling a flowrate of the solvent fed to said extractive distillation tower;controlling a reflux flow rate of part of a residual component of thefeedstock taken out from a top of the extractive distillation tower andrefluxed to the extractive distillation tower; controlling a bottomtemperature of the extractive distillation tower; and calculatingforecasted values of the concentration of the target unsaturatedhydrocarbon based on values detected by the impurity concentrationdetecting step and target material concentration detecting step andcontrolling the return flow rate, the flow rate of the solvent, thereflux flow rate, and the bottom temperature based on the forecastedvalues.
 7. The method of separation and purification of an unsaturatedhydrocarbon as set forth in claim 6, further comprising detecting adifferential pressure between a top and bottom of said extractivedistillation tower and calculating forecasted values of theconcentration of the specific impurity and the concentration of thespecific impurity and the concentration of the target unsaturatedhydrocarbon based on values detected by the impurity concentrationdetecting step, target material concentration detecting step, anddifferential pressure detecting step.
 8. The method of separation andpurification of an unsaturated hydrocarbon as set forth in claim 6 or 7,further comprising feeding said solvent to said extractive distillationtower from a position higher than a petroleum fraction feed means forfeeding a petroleum fraction containing said unsaturated hydrocarbon. 9.The method of separation and purification of an unsaturated hydrocarbonas set forth in claims 6 or 7, further comprising vaporizing a petroleumfraction containing said unsaturated hydrocarbon before being fed tosaid extractive distillation tower and then feeding the petroleumfraction to said extractive distillation tower.
 10. The method ofseparation and purification as set forth in claims 6 or 7, furthercomprising separating a fluid containing the target unsaturatedhydrocarbon taken out from the bottom of said extractive distillationtower into the unsaturated hydrocarbon and solvent.
 11. The method ofseparation and purification as set forth in claims 6 or 7, furthercomprising using a petroleum fraction containing a conjugated diene as apetroleum fraction containing the unsaturated hydrocarbon before beingfed to the extractive distillation tower.
 12. The method of separationand purification as set forth in claim 11, wherein said conjugated dieneis butadiene and the concentration of said specific impurity is theconcentration of cis-2-butene and/or the concentration oftrans-2-butene.
 13. The method of separation and purification as setforth in claims 6 or 7, further comprising controlling the bottomtemperature of said extractive distillation tower to become the boilingpoint of the solvent at the tower pressure.
 14. The method of separationand purification as set forth in claims 6 or 7, further comprisingdetecting the concentration of said target unsaturated hydrocarbon at atop of said extractive distillation tower.